Note: Descriptions are shown in the official language in which they were submitted.
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Sheet extrudates with self-cleaning properties and process for
producing these extrudates
The invention relates to sheet extrudates with self-cleaning properties and
to a process for their production.
Various processes for treating surfaces to render these surface dirt- and
water-repellent are known from the surface technology. For example, it is
known that if a surface is to have good self-cleaning properties it has not
l0 only to be hydrophobic but also to have a certain roughness. A suitable
combination of structure and hydrophobic properties permits even small
amounts of water set in motion on the surface to entrain adherent dirt
particles and clean the surface (VllO 96/04123; US 3354022, C. Neinhuis,
W. Barthlott, Annals of botany 79, (1997), 667).
As early as 1982, A.A. Abramson in Chimia i Shisn russ. 11, 38, described,
although without any recognition of self-cleaning properties, the roll-off of
water droplets on hydrophobic surfaces particularly if these have
structuring.
The prior art of EP 0 933 388 in relation to self-cleaning surfaces is that
these self-cleaning surfaces require an aspect ratio > 1 and a surface
energy smaller than 20 mN/m. The aspect ratio is defined here as the
quotient derived from the average height of the structure in relation to its
average width. The abovementioned criteria are realized in the natural
world, for example in the lotus leaf. The surface of the plant, formed from a
hydrophobic waxy material, has elevations separated from one another by
a few ~,m. To a substantial extent, water droplets come into contact only
with these peaks. There are many descriptions in the literature of water-
3o repellent surfaces of this type, an example being an article in
Langmuir 2000, 16, 5754, by Masashi Miwa et al., according to which the
contact angle and roll-off angle increase with an increase in the degree of
structuring of artificial surfaces formed from boehmite, applied to a spin-
coated layer and then calcined.
Swiss patent 268258 describes a process in which structured surfaces are
produced by applying powders, such as kaolin, talc, clay, or silica gel. The
powders are secured to the surface by oils and resins based on
,
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organosilicon compounds.
The use of hydrophobic materials, such as perfluorinated polymers, for
producing hydrophobic surfaces is known. DE 197 15 906 A1 states that
perfluorinated polymers, such as polytetrafluoroethylene, or copolymers of
polytetrafluoroethylene with perfluoro alkyl vinyl ethers produce
hydrophobic surfaces which have structuring and have low adhesion to
snow and ice. JP 11171592 describes a water-repellent product and its
production, the dirt-repellent surface being produced by applying, to the
l0 surface to be treated, a film which comprises fine particles of metal oxide
and comprises the hydrolyzate of a metal alkoxide or of a metal chelate. To
consolidate this film, the substrate to which the film has been applied has
to be sintered at temperatures above 400°C. This process is therefore
useful only for substrates which can be heated to temperatures above
400°C.
The processes usually . used hitherto for producing self-cleaning surfaces
are complicated and often have limited applicability. For example,
embossing techniques are inflexible for application of structures to three-
2 o dimensional bodies of various shapes. There is still no suitable
technology
for producing flat, large-surface-area coating films. Processes in which
structure-forming particles are applied to surfaces by means of a carrier -
for example an adhesive - have the disadvantage that the surfaces
obtained are composed of a very wide variety of combinations of materials
which, for example, have different coefficients of thermal expansion, and
this can lead to damage to the surface.
It was therefore an object of the present invention to provide a process for
producing self-cleaning surfaces on flat, large-surface-area moldings. The
3o method utilized here should be as simple as possible and resultant self-
cleaning surfaces should be durable.
Surprisingly, it has been found that particles can be securely bonded to the
surface of the sheet extrudate by applying hydrophobic, nanostructured
particles to a roll used for the smoothing of sheet extrudates. If the
particles
used have hydrophobic properties, they can at the same time act as a
release agent. To apply the particles it is preferable to use one or both of
the rolls located in the immediate vicinity of the die. At this location, the
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polymer melt emerging from the die has not solidified sufficiently to prevent
impression of the structured particles and bonding to the polymer matrix.
The present invention provides sheet extrudates with at least one surface
which has self-cleaning properties wherein the surface has at least one
securely anchored layer of microparticles which form elevations.
The present invention also provides a process for producing sheet
extrudates of the invention with at least one surface which has self-
1o cleaning properties and has elevations formed by microparticles, which
comprises using a roll to impress microparticles into the surface of a sheet
extrudate.
The present invention also provides films or sheets with a surface which
has self-cleaning properties and has surface structures with elevations, the
production process being the process of the invention.
The process of the invention has the advantage that to a very substantial
extent use can be made of existing equipment for producing sheet
extrudates. Sheet extrudates are usually smoothed by means of rolls. The
2 0 process of the invention makes use of these rolls by applying
microparticles to these rolls. The particles are impressed into the surface of
the extrudate and thus transferred thereto during smoothing of the
extrudates. This simple method gives access to sheet extrudates with self-
cleaning surfaces which comprise particles with a fissured structure,
without any need to apply an additional emboss layer or foreign material
carrier layer to the extrudate.
If the particles are hydrophobic particles, these at the same time act as a
release agent, since the powder applied to the roll prevents the material of
3 o the sheet extrudate from adhering to the roll used for smoothing.
The sheet extrudates of the invention have the advantage that structure
forming particles are not secured by a carrier material, thus avoiding any
unnecessarily high number of combinations of material and the adverse
properties associated therewith.
The process of the invention provides access to self-cleaning sheet
extrudates in which the self-cleaning properties are not the result of
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additional application of material, other than the application of particles,
or
of any additional chemical process.
Another advantage of the process of the invention is that surfaces
susceptible to scratching are not damaged by subsequent mechanical
application of a carrier layer and/or of particles.
A factor which is proving very particularly advantageous is that any desired
sizes of surface can be provided with self-cleaning properties on one or
both sides.
In addition, impairment of the flexibility of films is less marked than on
application of a carrier layer, and associated with this there is also no
substantial loss of secondary product properties.
The invention will be described by way of example below, but is not
restricted to these embodiments.
A feature of the sheet extrudates of the invention with at least one surface
which has elevations and has self-cleaning properties is that the surface
has at least one securely anchored layer of microparticles which form the
elevations. The elevations present on at least some of the surface of the
moldings combine with hydrophobic properties of the surfaces to ensure
that these regions of the surfaces have low wettability and therefore have
2 5 self-cleaning properties. The manner of obtaining the securely anchored
layer of microparticles is that microparticles are applied in the form of a
layer to a roll, and then this roll is used to impress and anchor the
microparticles into the sheet extrudate. Particularly stable anchoring is
obtained if use is made of microparticles whose surface has a fine
structure, since the fine structure is to some extent filled by the extrudate
melt, and many anchoring points are present once the extrudate melt has
solidified/hardened. For the purposes of the present invention, a layer of
microparticles is a surface accumulation of microparticles which form
elevations. The layer may have been formed in such a way that the surface
comprises exclusively microparticles, almost exclusively mircoparticles, or
else microparticles whose separation is from 0 to 10 particle diameters, in
particular from 0 to 3 particle diameters.
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The surfaces of sheet extrudates with self-cleaning properties preferably
have elevations with an average height of from 20 nm to 25 ~m and with an
average separation of from 20 nm to 25 Vim, preferably with an average
height of from 50 nm to 10 ~m and/or with an average separation of from
50 nm to 10 p,m, and very particularly preferably with an average height of
from 50 nm to 4 pm and/or with an average separation of from 50 nm to
4 Vim. The sheet extrudates of the invention very particularly preferably
have surfaces with elevations with an average height of from 0.25 to 1 ~.m
and with an average separation of from 0.25 to 1 Vim. For the purposes of
1 o the present invention, the average separation of the elevations is the
separation between the highest elevation of an elevation and the most
adjacent highest elevation. If an elevation has the shape of a cone, the tip
of the cone is the highest elevation of the elevation. If the elevation is a
rectangular parallelepiped, the uppermost surface of the rectangular
parallelepiped is the highest elevation of the elevation.
The wetting of bodies can be described via the contact angle formed by a
water droplet with the surface. An angle of contact of 0 degree here means
complete wetting of the surface. The static contact angle is generally
2 o measured using equipment in which the contact angle is determined
optically. On smooth hydrophobic surfaces, the static contact angles
measured are usually below 125°. The present moldings with self-
cleaning
surfaces have static contact angles which are preferably greater than
130°,
with preference greater than 140°, and very particularly preferably
greater
than 145°. In addition, it has been found that a surface has good self-
cleaning properties only when it exhibits a difference of not more than
10°
between advancing and receding angle, and for this reason surfaces of the
invention preferably have a difference of less than 10°, preferably
less than
5°, and very particularly preferably less than 4°, between
advancing and
3 o receding angle. To determine the advancing angle, a water droplet is
placed on the surface by means of a cannula, and the droplet is enlarged
on the surface by adding water through the cannula. During enlargement,
the margin of the droplet glides over the surface, and the contact angle
determined is the advancing angle. The receding angle is measured on the
same droplet, but water is removed from the droplet through the cannula,
and the contact angle is measured during reduction of the size of the
droplet. The difference between the two angles is termed hysteresis. The
smaller the difference, the smaller the interaction of the water droplet with
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the surface of the substrate, and therefore the better the lotus effect (the
self-cleaning effect).
The surfaces of the invention with self-cleaning properties preferably have
an aspect ratio greater than 0.15 for the elevations. The elevations formed
by the particles themselves preferably have an aspect ratio of from 0.3 to
0.9, particularly preferably from 0.5 to 0.8. The aspect ratio is defined here
as the quotient derived from the average height of the structure of the
elevations in relation to its average width.
to
A feature of the sheet extrudates of the invention, which have self-cleaning
properties and surface structures with elevations is that the surfaces are
synthetic polymer surfaces into which the particles have been directly
anchored, and have not been linked via carrier systems or the like.
The method of bonding or anchoring the particles to the surface is that rolls
are used to impress the particles into the sheet extrudate. In order to
achieve the specified aspect ratios it is advantageous for at least some of
the particles, preferably more than 50% of the particles, to be impressed
into the surface of the sheet extrudate to the extent of only 90% of their
2o diameter. The surface therefore preferably comprises particles which have
been anchored with from 10 to 90%, preferably from 20 to 50%, and very
particularly preferably from 30 to 40%, of their average particle diameter
within the surface, and which therefore still have parts of their inherently
fissured surface protruding from the extrudate. This method ensures that
2 5 the elevations which are formed by the particles themselves have a
sufficiently large aspect ratio, preferably at least 0.15. This method also
ensures that the securely bonded particles have very durable bonding to
the surface of the molding. The aspect ratio is defined here as the ratio of
maximum height to maximum width of the elevations. A particle assumed
30 to be ideally spherical and protruding to an extent of 70% from the surface
of the sheet extrudate has an aspect ratio of 0.7 according to this
definition.
The microparticles securely bonded to the surface and forming the
elevations on the surface of the sheet extrudates have preferably been
35 selected from silicates, minerals, metal oxides, metal powders, silicas,
pigments, and polymers, very particularly preferably from fumed silicas,
precipitated silicas, aluminum oxide, mixed oxides, doped silicates, titanium
dioxides, and pulverulent polymers.
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Preferred microparticles have a diameter of from 0.02 to 100 Vim,
particularly preferably from 0.01 to 50 pm, and very particularly preferably
from 0.1 to 30 pm. However, other suitable micro particles are those which
have a diameter of less than 500 nm or which combine primary particles to
give agglomerates or aggregates whose size is from 0.2 to 100 wm.
Particularly preferred microparticles which form the elevations of the
structured surface are those whose surface has an irregular fine structure
1 o in the nanometer range. These microparticles having the irregular fine
structure preferably have elevations/fine structures whose aspect ratio is
greater than 1, particularly preferably greater than 1.5. The aspect ratio is
again defined as the quotient obtained by dividing the maximum height of
the elevation by its maximum width. Fig. 1 illustrates diagrammatically the
difference between the elevations formed by the particles and the
elevations formed by the fine structure: The figure shows the surface of a
sheet extrudate X, which comprises particles P (only one particle being
depicted to simplify the illustration). The elevation formed by the particle
itself has an aspect ratio of about 0.71, calculated as the quotient obtained
from the maximum height of the particle mH, which is 5, since only that
portion of the particle which protrudes from the surtace of the sheet
extrudate X contributes to the elevation, and from the maximum width mB,
which in turn is 7. A selected elevation of the elevations E present on the
particles by virtue of the fine structure of the particles has an aspect ratio
of
2 5 2.5, calculated as quotient from the maximum height of the elevation mH',
which is 2.5, and from the maximum width mB', which in turn is 1.
Preferred microparticles whose surface has an irregular fine structure in the
nanometer range are particles which comprise at least one compound
3o selected from fumed silica, precipitated silicas, aluminum oxide, mixed
oxides, doped silicates, titanium dioxides, and pulverulent polymers.
It can be advantageous for the microparticles to have hydrophobic
properties, and the hydrophobic properties may be attributable to the
properties of the materials themselves present on the surfaces of the
35 particles, or else may be obtained through treatment of the particles with
a
suitable compound. The microparticles may be provided with hydrophobic
properties prior to or after application to the surface of the sheet
extrudate.
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To hydrophobicize the particles prior to or after application to the surface,
they may be treated with a compound suitable for hydrophobicization, e.g.
from the group of the alkylsilanes, the fluoroalkylsilanes, and the
disilazanes.
Highly preferred microparticles are described in more detail in the following.
The particles may derive from various fields. They may be silicates, for
example, or doped silicates, minerals, metal oxides, aluminum oxide,
silicas, or titanium dioxides, Aerosils, or pulverulent polymers, e.g. spray-
dried and agglomerated emulsions, or cryogenically milled PTFE.
Particularly suitable particle systems are hydrophobized fumed silicas,
known as Aerosils~. To generate the self-cleaning surfaces, hydrophobic
properties are needed in addition to the structure. The particles used may
themselves be hydrophobic, for example pulverulent
polytetrafluoroethylene (PTFE). The particles may have been provided with
hydrophobic properties, for example Aerosil VPR 411 ~ or Aerosil R 8200~'
However, they may also be hydrophobicized subsequently. It is
unimportant here whether the particles are hydrophobicized prior to
application or after application. Examples of particles to be hydrophobized
2 0 are Aeroperl 90/30~, Sipernat silica 350~, aluminum oxide C~, zirconium
silicate, vanadium-doped or Aeroperl P 25/20~. In the case of the latter, the
hydrophobicization advantageously takes place through treatment with
perfluoroalkylsilane compounds, followed by heat-conditioning.
2 5 The sheet extrudates may have the elevations on all, in particular en two,
surfaces, or only on certain surfaces. The moldings of the invention
preferably have the elevations on only one of the two surfaces.
The material of the sheet extrudates themselves may preferably comprise
30 polymers based on polycarbonates, on polyoxymethylenes, on
poly(meth)acrylates, on polyamides, on polyvinyl chloride (PVC), on
polyethylenes, on polypropylenes, on polystyrenes, on polyesters, on
aliphatic linear or branched polyalkenes, on cyclic polyalkenes, on
polyacrylonitrile, or on polyalkylene terephthalates, or else may comprise
35 their mixtures or copolymers. The material of the sheet extrudates
particularly preferably comprises a material selected from poly(vinylidene
fluoride), poly(hexafluoropropylene), poly(perfluoropropylene oxide), poly-
(fluoroalkyl acrylate), poly(fluoroalkyl methacrylate), polyvinyl
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perfluoroalkyl ether), or comprises other polymers from perfluoroalkoxy
compounds, poly(ethylene), poly(propylene), poly(isobutene), poly-
(4-methyl-1-pentene), or polynorbornene, in the form of homo- or
copolymer. The material for the surface of the sheet extrudates particularly
preferably comprises poly(ethylene), poly(propylene), polycarbonate,
polyesters, or poly(vinylidene fluoride). Besides the polymers, the materials
may comprise the usual additives and auxiliaries, e.g. plasticizers,
pigments, or fillers.
The sheet extrudates of the invention are preferably produced by the
process of the invention for producing sheet extrudates with at least one
surface which has self-cleaning properties and has elevations formed by
microparticles, which comprises using a roll to impress microparticles into
the surface of a sheet extrudate. The roll may be a roll specifically
provided. However, the roll used to impress the microparticles into the
surface of the melt of the sheet extrudate, where this melt has not yet
solidified, is particularly preferably a roll needed for the production of
conventional sheet extrudates, in particular a roll which is used for
smoothing sheet extrudates and which is in any case usually already
present. For applying the particles it is preferable to use one or two of the
rolls located in the immediate vicinity of the die. At this location, the
polymer melt emerging from the die has not yet solidified to an extent
which would prevent impression of the structured particles and bonding to
the polymer matrix.
The method of impression is preferably such that the extent to which the
particles are impressed into the surface of the sheet extrudate is only a
maximum of 90% of their diameter, preferably from 10 to 90%, with
preference from 20 to 50%, and very particularly preferably from 30 to
40%, of their average diameter.
The sheet extrudate used may be any of the sheet extrudates based on
polymers. Preference is given to sheet extrudates which comprise a
polymer based on polycarbonates, on polyoxymethylenes, on poly(meth)-
acrylates, on polyamides, on polyvinyl chloride, on polyethylenes, on
polypropylenes, on aliphatic linear or branched polyalkenes, on cyclic
polyalkenes, on polystyrenes, on polyesters, on polyacrylonitrile, or on
polyalkylene terephthalates, or on poly(vinylidene fluoride), or which
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comprise other polymers from poly(isobutene), poly(4-methyl-1-pentene),
and polynorbornene, in the form of homo- or copolymer, or a mixture of
these. Besides the polymers, the sheet extrudates may comprise the usual
additives and/or auxiliaries, e.g. plasticizers, pigments, or fillers.
The microparticles which in the process of the invention are impressed into
the surface of the melt of the sheet extrudate by means of a roll, where this
melt has not yet hardened, may be applied, prior to impression, either to
the surface of the extrudate or else to the surface of the roll used for
impression. If the microparticles are applied to the sheet extrudate,
application methods which may be used are spraying, scattering, or the
like. The microparticles are usually in loose form on application to the sheet
extrudate. It can also be advantageous for the microparticles to be applied
to the roll prior to impression. The method of application may be spraying
or scattering. The application of the microparticles to the roll can in
particular be advantageous because the microparticie powder applied to
the roll, in particular to the roll used for smoothing, prevents adhesion of
the material of the sheet extrudate to the roll during smoothing (and during
impression of the microparticles), since material does not usually come into
contact at all with the roll, because the microparticles have been applied
2 0 very densely to the roll to achieve the preferred separations of the
elevations. This release effect is naturally also achieved if the
microparticles are applied to the sheet extrudate. It can be advantageous
to apply the microparticles both to the sheet extrudate and to the roil.
An example of a way of applying the microparticles to the roll by spraying is
spray-application of microparticle powders or dispersions which comprise,
besides the microparticles, a solvent, which is preferably volatile. The
solvent present in the suspensions used preferably comprises an alcohol,
in particular ethanol or isopropanol, ketones, e.g. acetone or methyl ethyl
ketone, ethers, e.g. diisopropyl ether, or else hydrocarbons, such as
cyclohexane. The suspensions very particularly preferably comprise
alcohols. It can be advantageous for the suspension to comprise from 0.1
to 10% by weight, preferably from 0.25 to 7.5% by weight, and very
particularly preferably from 0.5 to 5% by weight, of microparticles, based on
the total weight of the suspension.
In particular for spray-application of a suspension, it can be advantageous
for the roll to have a temperature of from 20 to 150°C. Depending on
the
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sheet extrudate, however, the temperature of the roll may also be within
the specified range irrespective of the microparticles or of the application
of
the microparticles.
The pressure which the roll exerts on the sheet extrudate in order to
smooth the same and/or to press the microparticles into the surface of the
sheet extrudate cannot be determined and depends on the material to be
smoothed and its finish, and also on the width of the gap between the two
rolls which are used to smooth the extrudate. The width of the gap between
the rolls may be set as desired within wide limits.
Typical widths of the gap vary from a few micrometers as far as a number
centimeters, preferably from 5 pm to 5 cm. It is often found that the depth
of impression of the particles into the extrudate decreases with increasing
gap width. This is probably associated with the increasing flexibility of the
material as the extrudate becomes thicker. The process of the invention is
therefore preferably used with sheet extrudates whose material has a
thickness of from 5 p,m to 500 pm. The process of the invention can also
naturally produce sheets with a hollow chamber, e.g. sandwich panels. The
2 0 total thickness of the material of these may be markedly more than
500 wm. In order that the smoothing rolls do not compress the hollow
sheets in these processes, a superatmospheric pressure is produced in the
hollow chambers, e.g by compressed air, so that compression is very
substantially avoided.
It can be advantageous to use at least two rolls and to impress
microparticles into the surface of the sheet extrudate on two sides of the
sheet extrudate. It can be particularly advantageous for the microparticles
to be impressed by one of two, or else by two, opposite rolls between
which the sheet extrudate passes.
The microparticles used in the process of the invention are preferably those
which comprise at least one material selected from silicates, minerals,
metal oxides, metal powders, silicas, pigments, and polymers. It is
preferable to use microparticles whose diameter is from 0.02 to 100 p,m,
particularly preferably from 0.01 to 50 Vim, and very particularly preferably
from 0.1 to 30 Vim. It is also possible to use microparticles with diameters
less than 500 nm. However, other suitable microparticles are those which
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combine primary particles to give agglomerates or aggregates whose size
is from 0.2 to 100 pm.
Preferred microparticles used, in particular particles whose surface has an
irregular fine structure in the nanometer range, are particles which
comprise at least one compound selected from fumed silica, precipitated
silicas, aluminum oxide, mixed oxides, doped silicates, titanium dioxides,
and pulverulent polymers. Preferred particles whose surface has an
irregular fine structure in the nanometer range have, by virtue of this fine
1o structure, surface elevations which have an aspect ratio greater than 1,
particularly preferably greater than 1.5, and very particularly preferably
greater than 2.5. The aspect ratio is again defined as the quotient derived
from the maximum height of the elevation in relation to its maximum width.
The microparticles preferably have hydrophobic properties, and these
hydrophobic properties may be attributable to the properties of the
materials themselves present on the surfaces of the particles, or else may
be obtained through treatment of the particles with a suitable compound.
The particles may be provided with hydrophobic properties prior to or after
impression into the surface.
To hydrophobicize the microparticles prior to or after impression
(anchoring) into the surface of the sheet extrudate, these may be treated
with a compound suitable for hydrophobicization, e.g. from the group of the
alkylsilanes, the fluoroalkylsilanes, and the disilazanes.
Microparticles whose use is preferred are described in more detail in the
following. The particles used may derive from various fields. They may be
silicates, for example, or doped silicates, minerals, metal oxides, aluminum
oxide, silicas, or titanium dioxides, Aerosils~, or pulverulent polymers, e.g.
spray-dried and agglomerated emulsions, or cryogenically milled PTFE.
Particularly suitable particle systems are hydrophobized fumed silicas,
known as Aerosils~. To generate the self-cleaning surfaces, hydrophobic
properties are needed in addition to the structure. The particles used may
themselves be hydrophobic, for example pulverulent
polytetrafluoroethylene (PTFE). The particles may have been provided with
hydrophobic properties, for example Aerosil VPR 411 ~ or Aerosil R 8200~.
However, they may also be hydrophobicized subsequently. It is
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unimportant here whether the particles aare hydrophobicized prior to
application or after application. Examples of particles to be hydrophobized
are Aeroperl 90/30~, Sipernat silica 350~, aluminum oxide C~, zirconium
silicate, vanadium-doped or Aeroperl P 25/20~. In the case of the latter, the
hydrophobicization advantageously takes place through treatment with
perfluoroalkylsilane compounds, followed by heat-conditioning.
Examples of products which can be produced by means of the process of
the invention are sheets, including sheets with hollow chambers, and films,
where these have at least one surface having self-cleaning properties and
having surface structures with elevations. These films or sheets may be
applied to buildings, vehicles, or other articles, for example, so that these
likewise have self-cleaning properties. However, the films may also be
used as they stand, for example as packaging films which keep the
packaged product free from moisture and dirt.
The process of the invention is described using the examples below, but
there is no intention that the invention be restricted to these examples.
2 0 Example 1:
A polyoxymethylene (Ultraform~ W2320-003, BASF AG) sheet extrudate
with a thickness of 5 mil (1 mil corresponding to 25 ~,m) is dusted on one
side with hydrophobic fumed silica, Aerosil R 8200, Degussa AG, after
leaving the extruder (ZDSK28, Werner & Pfleiderer). The dusted extrudate
is smoothed by a roll pair located directly downstream of the dusting
apparatus and adjusted to a gap width of 5 mil. The solidified extrudate
obtained after treatment by the roll pair has particles impressed into the
surface of the extrudate on one side of the film, more than 70% of these
having been anchored with from 70 to 90% of their diameter within the
surface. The roll-off angle for a water droplet is determined on the resultant
extrudate surface by applying a droplet to the surface and constantly
increasing the inclination of the extrudate to determine the angle at which
the droplet rolls off from the surface. A roll-off angle smaller than
20° is
found for a water droplet of size 40 ~I.
Example 2:
After leaving the extruder a nylon-12 (Vestamid~ L1600, Degussa AG)
sheet extrudate with a thickness of 5 mil is passed through a gap between
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two rolls which serves to smooth the extrudate (ZDSK28, Weoner &
Pfleiderer), the gap width being adjested to 5 mil. The upper of the two rolls
is sprayed with hydrophobic fumed silica, Aerosil R 8200, Degussa AG,
suspended in ethanol. The roll presses these particles into the extrudate
during the smoothing procedure, where this extrudate has not yet solidified.
The solidified extrudate obtained after treatment by the roll has particles
impressed into the surface of the extrudate, more than 70% of these having
been anchored with from 70 to 90% of their diameter within the surface.
The roll-off angle for a water droplet is determined on the resultant
extrudate surface by applying a droplet to the surface and constantly
increasing the inclination of the extrudate to determine the angle at which
the droplet rolls off from the surface. A roll-off angle smaller than
30° is
found for a water droplet of size 40 ~I.
As can be seen from the examples, the process of the invention can give
extrudates which have self-cleaning or water-repellent surfaces, and it is of
almost no consequence here whether the microparticles are applied to the
roll or to the extrudate.
